In general, an active-matrix organic light emitting diode (AMOLED) display has the advantages of thin, lightweight, self-luminous, low driving voltage, high performance, high contrast ratio, high color saturation, quick response rate and flexibility, and thus the AMOLED display technology becomes the most promising emerging display technology after the thin film transistor liquid crystal display (TFT-LCD) technology has been introduced.
However, the brightness performance of the OLED is determined by the magnitude of current passing through the OLED, and the current must be controlled accurately to control the brightness of pixels accurately. Compared with the TFT-LCD that simply controls the voltage levels of written pixels to control the brightness of pixels, the OLED involves a higher level of difficulty.
In fact, the AMOLED has also encountered many problems. With reference to FIGS. 1 and 2 for schematic circuit diagrams of a p-type transistor and an n-type transistor of the AMOLED without compensation respectively, the current IOLED of the OLED is converted from data voltage VDATA by using the thin film transistor (TFT) T2 operated in a saturated region. For the n-type T2, the formula is IOLED=½*W/L*μN*COX(VGS−VTH)2, wherein after operating the AMOLED for a long time, the VTH of the TFT T2 will increase and the mobility μN will decrease, so that the IOLED will drop and cause the brightness of the OLED to decrease.
Furthermore, due to the material aging and the long-time operation of the OLED, the problems of a gradually increased voltage drop across the OLED and a decreased luminous efficiency may occur. The increase of voltage drop across the OLED may effect the operation of the TFT. As to the n-type TFT, if the OLED is coupled to a source of the n-type TFT, and the voltage drop across the OLED increases, both of the voltage between the source and the drain of TFT and the passing current will be affected directly. As to the luminous efficiency, the material aging and the intensity drop caused by the long-time operation will fail to produce the expected intensity even when a constant current is passed. If the luminous efficiencies of the red (R), green (G) and blue (B) colors drop differently, a color shift will occur. However, this problem cannot be solved easily, since improvement of the material cannot be made easily.
As the size of panels becomes larger and the length of the signal lines becomes increasingly longer, the internal resistance effect becomes more significant, and affects a uniform luminous efficiency of the panel, and such phenomenon is called an I-R Drop. With reference to FIG. 3 for a schematic view of the I-R Drop, the signal lines of VDD and VSS generate a dropout voltage by the internal resistance effect, so that the pixels disposed at different positions of the AMOLED panel have different currents that affect the uniform luminous efficiency of the panel.